Method of making a smooth low density paperboard

ABSTRACT

A paperboard product has at least two or three plies. One of the plies has at least 25% by weight bulky cellulosic fibers. Another of the plies is an outer ply and has an outer surface having a Parker PrintSurf measurement of between 3 and 6μ when measured at 10 kg/cm 2  using a soft backing surface. The paperboard product has a density no greater than 0.5 grams per cubic centimeter. An embodiment has a caliper in the range of 0.4 and 1.2 mm. An embodiment has a basis weight in the range of 200 to 500 grams per square meter. A method for making the paperboard product by passing the web through a soft nip calender and an extended nip calender which can be a shoe calender under conditions which provide the surface and density properties.

This invention relates generally to a low density paperboard containing bulky fibers and having a smooth surface. The invention also relates to calendering the paperboard to achieve these results.

Hot foods, particularly hot liquids, are commonly served and consumed in disposable containers. These containers are made from a variety of materials including paperboard and foamed polymeric sheet material. One of the least expensive sources of paperboard material is cellulose fibers. Cellulose fibers are employed to produce excellent paperboards for the production of hot cups, paper plates, and other food and beverage containers. Conventional paperboard produced from cellulosic fibers, however, is relatively dense, and therefore, transmits heat more readily than, for example, foamed polymeric sheet material. Thus, hot liquids are typically served in double cups or in cups containing multiple plies of conventional paperboard.

Low density insulating paperboard contains fibers which have the purpose of reducing the temperature drop across the paperboard. The paperboard would be made into a container which maintains the heat of the contained product and has an outer surface which is at a temperature that can be handled without discomfort. The materials within the insulating paperboard may by their nature be lumpy and not smooth. This can provide a surface that is not smooth and may be difficult to use for some types of printing.

It is desirable to have a low density paperboard that has a smooth surface because the containers are used by retail outlets that desire to have a business name and logo on the container. The outlets may also wish to have a statement about the temperature of the contained product. A smooth surface is more useful for printing.

A concern is how to maintain a low density board while providing a smooth surface. Will the pressures required to obtain a smooth surface increase the density unacceptably?

An embodiment of this invention is a paperboard that has a low density and a surface that is smooth. Another embodiment of this invention is a process for manufacturing a paperboard having a low density and a surface that is smooth.

Low density insulating paperboard may be in two or more plies. At least one of the plies contains both regular chemical bleached, semi-bleached or unbleached pulp fibers and bulky fibers. Bulky fibers give the ply insulative qualities because they provide bulk to the ply and create a lower density ply. The purpose of these bulky fibers is to provide a greater temperature drop across the thickness of the paperboard than would be provided if bulky fibers were not used. Another of the plies is a surface ply which does not contain bulky fibers and has a smooth outer surface.

Bulky fibers can be mechanically produced or can be produced by crosslinking the cellulosic fibers. The bulky fibers create a rough surface on the paperboard. It is necessary to smooth the surface of the paperboard while maintaining the low density of the paperboard. In an embodiment of the invention at least one surface of the paperboard is smooth. In an embodiment of the invention at least one surface of the paperboard has a Parker PrintSurf measurement of between 3 and 6μ at 10 S where 10 indicates a clamp force of 10 kilograms and S indicates the use of a soft backing to the tested paperboard.

Paperboard of the present invention may have a broad set of characteristics. In one embodiment its basis weight can range from 200 grams per square meter to 500 grams per square meter. In another embodiment its basis weight can range from 250 grams per square meter to 400 grams per square meter. In another embodiment the basis weight of the paperboard is equal to or greater than 250 grams per square meter. To achieve the insulating characteristics of the present invention, the paperboard may have a density of less than 0.5 g/cc. In another embodiment the density may be from 0.3 g/cc to 0.45 g/cc. In another embodiment the density may be from 0.35 g/cc to 0.40 g/cc. In an embodiment of the present invention the caliper of the paperboard is in the range of 0.4 to 1.2 mm after calendering.

FIG. 1 is a schematic cross-sectional view of a two-ply paperboard.

FIG. 2 is a schematic cross-sectional view of a three-ply paperboard

The paperboard of the present invention has at least two plies. FIG. 1 illustrates a two-ply construction in which paperboard 10 has plies 12 and 14. FIG. 2 illustrates a three-ply construction in which the paperboard 20 has plies 22, 24 and 26. While the paperboard of the present invention may employ synthetic fibers, it would usually comprise all or substantially all cellulosic fibers.

The plies 12, 22 and 26 are of conventional hardwood or softwood cellulosic fibers. They may be bleached or unbleached. They may be made of chemical, mechanical, chemimechanical or thermomechanical pulp.

The plies 14 and 24 contain bulky fibers. The bulky fibers increase the bulk density of the paperboard and thus the insulating characteristics. As used herein, bulky fibers are kinked, twisted, curly, cellulosic fibers. It is preferred, however, that the fibers be produced by intrafiber crosslinking of the cellulosic fibers as described in more detail below. Conventional hardwood or softwood cellulosic fibers may be used with the bulky fibers in these plies.

Chemically crosslinked cellulosic fibers are suitable for use as the bulky fibers in paperboard. Any one of a number of crosslinking agents and crosslinking catalysts, if necessary, can be used to provide the crosslinked fibers to be included in the layer. The following is a representative list of useful crosslinking agents and catalysts.

Suitable urea-based crosslinking agents include substituted ureas, such as methylolated ureas, methylolated cyclic ureas, methylolated lower alkyl cyclic ureas, methylolated dihydroxy cyclic ureas, dihydroxy cyclic ureas, and lower alkyl substituted cyclic ureas. Specific urea-based crosslinking agents include dimethyldihydroxy urea (DMDHU, 1,3-dimethyl-4,5-dihydroxy-2-imidazolidinone), dimethyloldihydroxyethylene urea (DMDHEU, 1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone), dimethylol urea (DMU, bis[N-hydroxymethyl]urea), dihydroxyethylene urea (DHEU, 4,5-dihydroxy-2-imidazolidinone), dimethylolethylene urea (DMEU, 1,3-dihydroxymethyl-2-imidazolidinone), and dimethyldihydroxyethylene urea (DMeDHEU or DDI, 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone).

Suitable crosslinking agents also include dialdehydes such as C₂-C₈ dialdehydes (e.g., glyoxal), C₂-C₈ dialdehyde acid analogs having at least one aldehyde group, and oligomers of these aldehyde and dialdehyde acid analogs. Other suitable crosslinking agents include aldehyde and urea-based formaldehyde addition products, glyoxal adducts of ureas and glyoxal/cyclic urea adducts.

Other suitable crosslinking agents include carboxylic acid crosslinking agents such as polycarboxylic acids. Polycarboxylic acid crosslinking agents include citric acid, propane tricarboxylic acid, and butane tetracarboxylic acid) and catalysts. C₂-C₉ polycarboxylic acids that contain at least three carboxyl groups (e.g., citric acid and oxydisuccinic acid) are also suitable as crosslinking agents.

Polymeric polycarboxylic acids are also suitable crosslinking agents. Polyacrylic acid and related copolymers may be used as crosslinking agents. Polymaleic acid may be used as a crosslinking agent.

Specific suitable polycarboxylic acid crosslinking agents include citric acid, tartaric acid, malic acid, succinic acid, glutaric acid, citraconic acid, itaconic acid, tartrate monosuccinic acid, maleic acid, polyacrylic acid, polymethacrylic acid, polymaleic acid, polymethylvinylether-co-maleate copolymer, polymethylvinylether-co-itaconate copolymer, copolymers of acrylic acid, and copolymers of maleic acid.

Suitable crosslinking catalysts can include acidic salts, such as ammonium chloride, ammonium sulfate, aluminum chloride, magnesium chloride, magnesium nitrate, and alkali metal salts of phosphorous-containing acids. In one embodiment, the crosslinking catalyst is sodium hypophosphite.

The crosslinking agent is applied to the cellulosic fibers as they are being produced in an amount sufficient to affect intrafiber crosslinking. The amount applied to the cellulosic fibers may be from about 1% to about 25% by weight based on the total weight of fibers. In one embodiment, crosslinking agent in an amount from about 4% to about 6% by weight based on the total weight of fibers. Mixtures or blends of crosslinking agents and catalysts can also be used.

The insulative fibers may also be mechanically pulped fibers, thermomechanical pulped fibers, chemithermomechanically pulped fibers or combinations of these.

Insulating fibers, for the most part, are not smooth. They often are curled and twisted. They have a tendency to provide an uneven surface to the low density insulative paperboard.

In addition to fibrous materials, the paperboard of the invention may optionally include a binding agent. Suitable binding agents are soluble in, dispersible in, or form a suspension in water. Suitable binding agents include those agents commonly used in the paper industry to impart wet and dry tensile and tearing strength to such products. Suitable wet strength agents include cationic modified starch having nitrogen-containing groups (e.g.,. amino groups), such as those available from National Starch and Chemical Corp., Bridgewater, N.J.; latex; wet strength resins, such as polyamide-epichlorohydrin resin (e.g., KYMENE 557LX, Hercules, Inc., Wilmington, Del.), and polyacrylamide resin (see, e.g., U.S. Pat. No. 3,556,932 and also the commercially available polyacrylamide marketed by American Cyanamid Co., Stanford, Conn., under the trade name PAREZ 631 NC); urea formaldehyde and melamine formaldehyde resins; and polyethylenimine resins. A general discussion on wet strength resins utilized in the paper field, and generally applicable in the present invention, can be found in TAPPI monograph series No. 29, “Wet Strength in Paper and Paperboard”, Technical Association of the Pulp and Paper Industry (New York, 1965).

Other suitable binding agents include starch, modified starch, polyvinyl alcohol, polyvinyl acetate, polyethylene/acrylic acid copolymer, acrylic acid polymers, polyacrylate, polyacrylamide, polyamine, guar gum, oxidized polyethylene, polyvinyl chloride, polyvinyl chloride/acrylic acid copolymers, acrylonitrile/butadiene/styrene copolymers, and polyacrylonitrile. Many of these will be formed into latex polymers for dispersion or suspension in water.

The insulative fibers may be blended with other cellulosic fibers which have lesser insulative properties. Although available from other sources, these other fibers are derived primarily from wood pulp. Suitable wood pulp fibers for use with the invention can be obtained from well-known chemical processes such as the kraft and sulfite processes, with or without subsequent bleaching. The preferred pulp fiber is produced by chemical methods. Recycled or secondary wood pulp fibers, and bleached and unbleached wood pulp fibers can be used. Softwoods and hardwoods can be used. Details of the selection of wood pulp fibers are well known to those skilled in the art. These fibers are commercially available from a number of companies, including Weyerhaeuser Company, the assignee of the present invention. For example, suitable cellulose fibers produced from southern pine that are usable with the present invention are available from Weyerhaeuser Company under the designations CF416, NF405, PLM16, FR516, and NB416. These are the same fibers that would be used in plies 12, 22 and 26.

Paperboard of the present invention may have a broad set of characteristics. In one embodiment its basis weight can range from 200 grams per square meter to 500 grams per square meter. In another embodiment its basis weight can range from 250 grams per square meter to 400 grams per square meter. In another embodiment the basis weight of the paperboard is equal to or greater than 250 grams per square meter. To achieve the insulating characteristics of the present invention, the paperboard may have a density of less than 0.5 g/cc. In another embodiment the density may be from 0.3 g/cc to 0.45 g/cc. In another embodiment the density may be from 0.35 g/cc to 0.40 g/cc.

In a two-ply structure, for example, the first ply may contain 100% non-bulky fibers by weight while the second ply may in one embodiment contain from 25% to 100% bulky fibers by weight and in another embodiment from 30% to 70% bulky fibers by weight. In a three-ply layer, for example, the bottom and top plies may comprise 100% of non-bulky fibers by weight while the middle ply contains in one embodiment from about 25% to about 100% bulky fiber by weight and in another embodiment from about 30% to about 70% bulky fibers by weight.

In each of these embodiments it is desired to have a surface which is smooth and printable. The surface would have a Parker PrintSurf (TAPPI Test Method T 555 om-04) of between 3 and 6μ at a clamping pressure of 10 kg/cm² and using a soft backing sheet.

It is understood that TAPPI test method T-555 is a recognized method of measuring surface smoothness. It is also understood that Collaborative Testing Services, Inc., 21331 Gentry Drive, Sterling, Va. 20166 has over the years checked variations in the readings among a population of Parker PrintSurf test machines using a clamping pressure of 10 kg/cm² and a standard test sample and have found variations of as much as ±1μ from a mean measurement. Therefore the measurement of between 3 and 6μ as used here and in the claims is a mean measurement and the actual measurement would be between 3 and 6μ±1μ.

The paperboard of the present invention can be formed using conventional papermaking machines including, for example, Rotoformer, Fourdrinier, inclined wire Delta former, and twin-wire forming machines.

A multi-ply paperboard may be made by using multiple headboxes arranged sequentially in a wet-forming process, or by a baffled headbox having the capacity of receiving and then laying multiple pulp furnishes. In a multi-ply construction the printing surface ply may be of other fibers and the non printing ply of a two ply construction or a middle ply of a three or more ply construction may be of insulative fibers or a blend of insulative fibers and other fibers.

Multilayer liquid packaging board basestock can be formed on a papermachine capable of producing multilayer product. The papermachine can be a conventional Fourdrinier machine fitted with multiple head boxes. Each of the plies would be placed on the Fourdrinier machine by one of the headboxes to form a multiply web. Vacuum would be applied to remove water from the web. The web would then be pressed in a conventional press section to remove additional water.

The paperboard web would then be dried. The dried web may then optionally pass through a size press of the puddle or metering type, or a blade coater, where additional chemicals may be applied to the web.

Materials that can be added at the size press are starch, polyvinyl alcohol, pigments such as calcium carbonate or clay, or lubricants that are compatible with the starch and other binders. The web then passes through another dryer section.

When bulky fibers are used in paperboard in accordance with the present invention, it has been found that the paperboard exiting the papermaking machine can be compressed to varying degrees. In one embodiment of the present invention the caliper of the paperboard is in the range of 0.4 to 1.2 mm after calendering.

Following the size press treatment and drying, the paperboard is passed through a calender station. In the calender station the paperboard is calendered first with a soft nip calender and then with an extended nip or shoe calender.

In soft nip calendering the web is pressed against a hot surface in a nip by a roll that has a resilient cover. The resilient cover gives the paper a longer dwell time in the nip compared to hard steel nips and also allows the smoothness and gloss development to occur at relatively uniform density across the width of the paper. In the soft nip calender the paperboard will be exposed to a heated surface having a temperature of 120° C. to 160° C. and be under a linear load of 40 to 150 N/mm.

Extended nip calendering uses an endless band/belt over a backing roll to provide support for the paper web that is pressed against a heated cylinder. A variation to this concept, shoe calendering, uses a shoe instead of a roll as a backing for the paperboard. The backing shoe provides longer nip widths and hence an increased dwell time. In either configuration the nip would have a width of 6 to 12 cm for best results. In the calender the paperboard will be exposed to a heated surface having a temperature of 180° C. to 250° C. and be under a linear load of 100 to 300 N/mm.

In the following examples, the paperboard was formed at a forming pilot facility, and calendered at a calendering pilot facility. The paperboard was three-ply having bulky fiber in the center ply. The paperboard had 20% of its fiber in each of the outer plies and 60% of its fiber in the center ply. The bulky fiber was 50% of the weight of the center ply. The bulky fiber was chemically intrafiber crosslinked cellulose fiber which used bleached chemical cellulosic wood pulp as its starting material. The fiber in the outer plies and the remainder of the fiber in the center ply was bleached chemical cellulosic wood pulp. Top Basis Calendered Softnip Softnip Shoe Shoe PPS weight Caliper Temp Load Temp Load 10 S Ex. g/m² mm ° C. N/mm ° C. N/mm μ 1 392 1.12 140 60 210 150 5.43 2 392 1.12 140 60 210 250 4.24 3 392 1.09 140 120 210 150 5.22 4 392 1.09 140 120 210 250 4.06

The foregoing invention has been described in conjunction with a preferred embodiment and various alterations and variations thereof. One of ordinary skill will be able to substitute equivalents in the disclosed invention without departing from the broad concepts imparted herein. It is therefore intended that the present invention be limited only by the definition contained in the appended claims 

1. A method of manufacturing a paperboard product comprising the steps of: wet laying at least two plies of pulp to form a web, one of said plies comprising a least 25% by weight bulky cellulosic fibers, another of said plies being an outer ply of said paperboard product and having an outer surface, removing water from said web, passing said web through a soft nip calender, said calender providing a temperature in the range of 120° C. to 160° C. to the web surface and a linear load in the range of 40 to 150 Newtons per millimeter, and thereafter passing said web through an extended nip calender having an extended nip in the range of 6 to 12 cm., a temperature in the range of 180° C. to 250° C. on the web surface and a linear load of 100 to 300 Newtons per millimeter to provide a paperboard product having a density no greater than 0.5 grams per cubic centimeter and said outer surface of said another ply having a Parker PrintSurf measurement of between 3 and 6μ when measured using a clamping pressure of 10 kg/cm² and a soft backing surface.
 2. The method of claim 1 in which said method provides a paperboard product having a caliper in the range of 0.4 and 1.2 mm.
 3. The method of claim 1 in which said method provides a paperboard product having a basis weight of 200 to 500 grams per square meter.
 4. The method of claim 1 in which said method provides a paperboard product having a density of between 0.3 and 0.45 grams per cubic centimeter.
 5. The method of claim 4 in which said method provides a paperboard product having a caliper in the range of 0.4 and 1.2 mm.
 6. The method of claim 4 in which said method provides a paperboard product having a basis weight of 200 to 500 grams per square meter.
 7. The method of claim 1 in which said method provides a paperboard product having a density of between 0.35 and 0.4 grams per cubic centimeter.
 8. The method of claim 7 in which said finished web has a caliper in the range of 0.4 and 1.2 mm.
 9. The method of claim 7 in which said method provides a paperboard product having a basis weight of 200 to 500 grams per square meter.
 10. The method of claim 1 in which there is a drying step between said soft nip calendar and said extended nip calender.
 11. The method of claim 1 in which said extended nip calendar is a shoe calender. 